Jack-up platform having a submerged tank and methods for installing and raising the tank

Abstract

A jack-up platform of the type having a buoyant upper barge ( 10 ) mounted so that it can be moved along the length of bearing legs ( 12 ), mechanical mechanism ( 18 ) for moving the legs ( 12 ) relative to the barge ( 10 ), and submerged storage tank ( 14 ) intended to rest on the bottom. The tank ( 14 ) has a lower opening ( 26 ) allowing the inside of the tank ( 14 ) to be placed in contact with the marine environment. The tank defines a bell-shaped/inverted U-shaped cavity confining an air bubble ( 30 ) in its upper part. The tank ( 14 ) is connected to the lower end of the legs ( 12 ) so that it can be moved relative to the barge ( 10 ) from the mechanical mechanism ( 18 ) for moving the legs.

What is claimed is:  
     
       1. A jack-up platform comprising: 
       a plurality of bearing legs having lower ends, respectively;  
       a buoyant upper barge mounted so as to be movable along the length of said bearing legs;  
       mechanical moving means for moving said bearing legs relative to said upper barge; and  
       a submersible storage tank connected to the lower ends of said bearing legs so that said storage tank can be moved by said mechanical means relative to said upper barge,  
       said storage tank having a lower opening for selectively permitting an interior of said storage tank to be placed in free communication with a body of water, said storage tank being adapted to rest on a surface below the body of water,  
       wherein an upper portion of said storage tank defines a cavity having an inverted U-shaped cross-section for confining an air pocket therein, and  
       wherein said mechanical means are operable to apply a force on said bearing legs in order to cause said storage tank to sink, and to apply an opposite force on said bearing legs to cause said storage tank to rise.  
     
     
       2. A jack-up platform as claimed in claim  1 , further comprising means for injecting gas under pressure into said storage tank. 
     
     
       3. A jack-up platform as claimed in claim  1 , wherein said storage tank comprises: 
       a base having a surface area that is at least equal to a surface area of said upper barge; and  
       at least one upper shield partially surrounding said base and extending along said upper barge when said upper barge is in close proximity to said base.  
     
     
       4. A jack-up platform as claimed in claim  3 , wherein said shield forms part of said storage tank. 
     
     
       5. A jack-up platform as claimed in claim  1 , wherein said storage tank comprises: 
       a base having a surface area that is at least equal to a surface area of said upper barge; and  
       a plurality of upper shields partially surrounding said base and extending along said upper barge when said upper barge is disposed in close proximity to said base,  
       wherein said storage tank defines a substantially polygonal shape, and said shields extend along most of the length of at least two sides of said base.  
     
     
       6. A jack-up platform as claimed in claim  1 , further comprising a valve positioned in said lower opening of said storage tank. 
     
     
       7. A jack-up platform as claimed in claim  1 , further comprising: 
       at least one sole member; and  
       at least one retainer securing said sole member to an underside of said storage tank, such that, when said storage tank is resting on the surface below the body of water via said sole member, said retainer can be released to allow said storage tank to rise without said sole member.  
     
     
       8. A jack-up platform as claimed in claim  7 , wherein said sole member covers a portion of the underside of said storage tank and the surface area of the underside covered by said sole member is less than half of the total surface area of the underside of said storage tank. 
     
     
       9. A jack-up platform as claimed in claim  7 , wherein said sole member, when attached to said storage tank, substantially conforms to an adjacent external contour of said storage tank, said sole member being positioned such that a central region of said storage tank is exposed. 
     
     
       10. A jack-up platform as claimed in claim  7 , wherein a height of said sole member maintaining a space between the surface below the body of water and a region of the underside of said storage tank that is not covered with said sole member. 
     
     
       11. A jack-up platform as claimed in claim  1 , further comprising: 
       a plurality of sole members adapted to rest on the surface below the body of water;  
       a plurality of retainers securing said sole members, respectively, to an underside of said storage tank, such that, when said storage tank is resting on the surface below the body of water via said sole members, said retainers can be released to allow said storage tank to rise in the body of water without said sole members attached thereto,  
       wherein said sole members cover a portion of the underside of said storage tank so that the surface area of the underside that is covered by said sole members is less than half of the total surface area of the underside of said storage tank,  
       wherein said sole members, when attached to said storage tank, generally conform to adjacent external contours of said storage tank, said sole members being positioned such that a central region of said storage tank is exposed; and  
       wherein a height of each of said sole members maintaing a space between the surface below the body of water and a region of the underside of said storage tank that is not covered with said sole members.  
     
     
       12. A jack-up platform as claimed in claim  11 , wherein said sole members are disposed so that said upper barge nests with said storage tank when said legs are in a raised position relative to said upper barge. 
     
     
       13. A method of installing a submerged tank of a jack-up platform that includes a plurality of bearing legs, a buoyant upper barge mounted so as to be movable along the length of the bearing legs; mechanical moving means for moving the bearing legs relative to the upper barge; and a submersible storage tank connected to the bearing legs so that the storage tank can be moved by the mechanical means relative to the upper barge, the method comprising: 
       placing the storage tank in free communication with a marine environment via a lower opening so that the pressure of air trapped within the tank and the pressure of water reach equilibrium, wherein said tank initially only has its own weight acting thereon;  
       progressively lowering the legs relative to the upper barge, which is kept constantly afloat, by action of the mechanical means and by gradually allowing the water to enter the storage tank through the lower opening such that the pressure in the storage tank is kept substantially equal to the hydrostatic pressure at any depth of the storage tank; and  
       jacking up the upper barge above an upper surface of the water once the storage tank is supported on a surface below the water, wherein the upper barge is jacked up above the upper surface of the water by moving the upper barge relative to the legs.  
     
     
       14. The method as claimed in claim  13 , wherein the method further comprises injecting gas into the storage tank before the legs are lowered relative to the upper barge. 
     
     
       15. The method as claimed in claim  13 , wherein, as the legs are being lowered during an initial phase, the mechanical means are operated to force the legs downward, and, during a later phase, the mechanical means are operated to restrain the legs from descending with the tank. 
     
     
       16. The method as claimed in claim  13 , further comprising pausing the lowering operation of the legs in order to allow the pressure inside the storage tank and the pressure outside of the storage tank to reach equilibrium by the water entering the storage tank. 
     
     
       17. A method of raising a submerged tank of a jack-up platform having a plurality of bearing legs, a buoyant upper barge mounted so as to be movable along the length of the bearing legs, mechanical moving means for moving the bearing legs relative to the upper barge, and a submersible storage tank connected to the bearing legs so that the storage tank can be moved by the mechanical moving means relative to the upper barge, the method comprising: 
       placing the storage tank in free communication with a body of water via a lower opening in the storage tank such that air is trapped in an air pocket inside of the storage tank, wherein the pressure of the air in the air pocket is equal to the hydrostatic pressure at the depth of the storage tank; and  
       raising the bearing legs gradually relative to the upper barge, which is kept constantly afloat, by action of the mechanical means and by allowing water to gradually leave the storage tank, thus keeping the pressure in the storage tank substantially equal to the hydrostatic pressure at the depth of the storage tank.  
     
     
       18. The method as claimed in claim  17 , further comprising injecting gas into the storage tank prior to the raising of the legs. 
     
     
       19. The method as claimed in claim  17 , further comprising controlling the volume of the air pocket such that, when raising the legs during an initial phase, the mechanical means are operating to force the legs upwards, and, during a later phase, the mechanical means are operating to restrain the raising of the legs which are being thrust upwardly by the storage tank. 
     
     
       20. The method as claimed in claim  17 , further comprising pausing the operation of raising the legs so as to allow the pressure inside of the storage tank and the pressure outside of the storage tank to reach equilibrium due to the water leaving the storage tank. 
     
     
       21. The method as claimed in claim  17 , further comprising releasing a plurality of sole members secured to a lower surface of said storage tank to permit the storage tank to rise without the sole members.